python

Handling directory locations
Error Handling
getting list of numbers as input in python
lambda functions
- list comprehension
- for loop
map function

A programming language. Using elpy as IDE in Emacs. Beancount has been written in python and i learnt python specifically to use this accounting software.

Initially i used Elpy as IDE in Emacs. Later decided to use inbuilt python mode.

For intial learning, i used Byte of Python book.

After that i used the https://programming-22.mooc.fi/ for learning. Could not finish the curriculum, particularly the later stages after the basics. Kept referring to AI tools, finally aborted.

Now on Monday 2025-10-27, starting again to learn.

In the recent attempt, i tried to follow book py4e. here are sample programs and concepts from that book.

Handling directory locations

import os
import sys

# Print the directory where Python is looking for files (the CWD)
print("Current Working Directory:", os.getcwd())

# Get the directory of the current script file
# __file__ is the full path of the script, os.path.dirname gets its directory
script_dir = os.path.dirname(os.path.abspath(__file__))

# Change the Current Working Directory to the script's directory
os.chdir(script_dir)

print("New Current Working Directory:", os.getcwd())
# Now it should be /home/prabu/py/py4e
name = input("Enter file: ")

try:
    # Python will now look for 'wordlist' inside /home/prabu/py/py4e/
    handle = open(name, 'r')
    # ... rest of your code
except FileNotFoundError:
    print(f"Error: Could not find '{name}' in {os.getcwd()}")
    sys.exit(1)

Error Handling

Always use focused and specific error handling for risky operations inside try..except blocks. Program logic that is generally safe can run outside of try blocks.

try:
    # 1. Risky operation: File I/O
    with open('config.txt', 'r') as f:
        config_data = f.read()
except FileNotFoundError:
    print("Error: Configuration file not found. Using defaults.")
    config_data = "DEFAULT"
except PermissionError:
    print("Error: Cannot read the configuration file due to permissions.")
    config_data = "DEFAULT"


try:
    # 2. Risky operation: Network access
    response = requests.get(URL)
    response.raise_for_status() # Raise HTTPError for bad responses
except requests.exceptions.HTTPError as e:
    print(f"HTTP Error: {e}")
except requests.exceptions.ConnectionError:
    print("Connection failed. Check network.")

# Program logic that is generally safe can run outside of try blocks
process_data(config_data, response.json())

getting list of numbers as input in python

user_input = input("Enter numbers separated by spaces: ") user_input = "10 20 30"

use one of the below method to create a list:

print(number_list)

lambda functions

A lambda is an anonymous function, often used with higher-order functions like map(), filter(), and sorted().

list comprehension

number_list = [int(x) for x in user_input.split()]

For lambda function, list comprehensions are often faster than map function.

for loop

Equivalent multi-line code using a for loop instead of list comprehension

number_list = []
for x in user_input.split():
    number_list.append(int(x))

map function

# Split the input string and convert each item to an integer
number_list = list(map(int, user_input.split()))